Code controlled broadcasting system

ABSTRACT

A radio broadcasting system includes a transmitter which transmits a program signal such as music on one channel, periodic message signals such as advertising commercials on another channel and a plurality of code signals of different frequencies occuring in time at the beginning of selected ones of the message signals. A plurality of radio receivers each receive the program, message and code signals and include means for audibly reproducing the program and message signals. Each receiver includes first and second filtering means responsive to only the program or message signals, respectively, together with switching means for connecting either of the first and second filtering means in the receiver circuit. Each receiver also includes a control circuit operative in response to the presence of a particular one of the code signals to cause movement of the switching means from one of the filters to the other. The control circuit includes feedback adjusting means in the frequency responsive input portion thereof, a high resistance coupling from input to output, and time delay means associated therewith which together assure that receiver operation is switched from program to message signal reproduction only in response to the particular intended code signal and not in response to other control signals closely spaced in frequency or to transient or spurious signals.

United States Patent Rogalski 1451 Jan. 30, 1973 [54] CODE CONTROLLED BROADCASTING [57] ABSTRACT SYSTEM A radio broadcasting system includes a transmitter [75] inventor: Theodore Rogalski, Buffalo, NY which transmits a program signal such as music on one [73] Assignee: Amalgamaed Music Enterprises, channel, periodic message signals such as advertising Inc. S racuse NY. commercials on another channel and a plurality of code signals of different frequencies occurmg in time [22] Filed: Aug. 7, 1970 at the beginning of selected ones of the message si nals. A luralit of radio receivers each receive the [211 App]' 61945 pr ogram, n iessage and code signals and include means for audibly reproducing the program and message [52] U.S. Cl. ..325/53, 325/53, 325/55, signals. Each receiver includes first and second filter- 325/392 ing means responsive to only the program or message [51] Int.Cl. ..l-l04h 1/04 g als, respectively, together with switching means [58] Fi ld f S h 325/51, 53, 54, 55, 57, 64, for connecting either of the first and second filtering 325 392, 4 30g; 179 15 3 5 means in the receiver circuit. Each receiver also in- 340/311; 343/200, 228; 330/21 26 31 cludes a control circuit operative in response to the presence of a particular one of the code signals to [56] R fe Cit d cause movement of the switching means from one of the filters to the other. The control circuit includes UNITED STATES PATENTS feedback adjusting means in the frequency responsive 2,980,794 4/1961 Hargreaves etal.... ....325/392 x input Portion thereof a l resistance f 3,162,820 12/1964 Horowitz .....330/31x Output and delay means assocated 3233136 2/1966 Theriauh i X therewith which together assure that receiver opera- 3,332,021 7/1967 Hedland ..325/392 tion is switched from p g to message signal 3,550,027 12/1970 Uetrecht ..330/26 reproduction only in response to the particular in- 2,709,254 /1955 Halstead ...179/l5 FD tended code signal and not in response to other con- 2,330,241 9/19 3 Roberts... trol signals closely spaced in frequency or to transient 2,617,923 I Rekart X o purious ignals 3,173,128 3/1965 Tatsazawa... ..340/l64 1 2,429,607 10/1947 Capen ..343/200 Claims, 6 Drawing Figures Primary ExaminerBenedict V. Safourek Attorney Christel & Bean 1 I :21: AMP LE 1.1mm: s, AUDIO 05c. M 1x [12 AMP D15Cl2lM1NATOQ AMP CONTROL 2O ClRCUlT PATENTED JAN 3 0 I975 SHEET 10F 6 mvsmm THEODOQE QOGALSK/ A Trap/v5 vs RF. AMP

osc MIXER INVENTOR.

THEODOQE QOGALSKI wwii ATTQQNEY PATENIEDJAHO I975 3,714,575

sum 30F e FIG. 3).

INVENTOR.

ATTORNEYS THEODORE QOGAL SKI PATENTEDJAHO ma 3,714,575 SHEET snr 6 INVENTOR; THEODORE ROGALSKi ATTOQNEYS FATENTEDJAN 30 I975 SHEET 6 OF 6 INVENTOR. THEODOQE ROGALSKI' ATTORNEYS CODE CONTROLLED BROADCASTING SYSTEM BACKGROUND OF THE INVENTION The present invention relates to radio broadcast systems, and, more particularly, to a system including a single transmitter and a plurality of receivers for broadcasting a program on one channel to all receivers and periodic messages on another channel to selected ones of the receivers, selection being accomplished by coded tones generated along with the particular messages.

One area of use of the present invention is in the broadcasting of background music simultaneously to a large number of business establishments for the pleasure of the customers therein and interrupting the music periodically with speech messages, such as advertising, at selected ones of groups of establishments to provide the customers with information. In recent times there has been a significant growth in the broadcasting of background music to business establishments such as supermarkets or banks. The geographic area included within the normal broadcast range of available equipment generally will contain several supermarket chains each having a considerable number of branches, or likewise, several banks each having a large number of branches. The proprietor or manager of each establishment would like to interrupt the music periodically and substitute an advertising message. With equipment heretofore available, thisof course can be done at each location by turning off the broadcast equipment and by using separate audio equipment to broadcast the commercial.

It would, therefore, be highly desirable to provide a radio system with the capability of broadcasting background music along with periodic commercial messages, the commercial being received by selected ones of the receivers and selection be accomplished by coded control signals. One requirement which is immediately apparent is proper routing of commercial messages to the various receivers because of the obvious undesirability of having an advertising message intended for one establishment being improperly received by an entirely different establishment. Such a system might be constructed so that any failure in the portion thereof which provides the routing of messages would result in continued transmission of the music rather than improper routing of the messages. In addition, the larger the number of radio receivers in a given system the closer will be the frequency spacing between the coded control signals thus making more difficult the design of the control signal responsive portions thereof. In particular, that portion of the receiver must have a high degree of sensitivity whereby it causes switching of receiver operation from program to message reception only in response to the occurrence of a particular one of the code signals.

SUMMARY OF THE INVENTION The present invention provides a radio broadcast system wherein there is transmitted from a single transmitting station a program signal such as music at a first frequency, periodic message signals such as commercials at a second frequency, and a plurality of code signals at different frequencies and occurring in time in correspondence with the beginning of selected ones of the message signals. Each of a plurality of receivers normally audibly reproduces the program signal and is provided with a frequency responsive control circuit which in response to the occurrence of a selected one of the code signals switches the receiver operation to audible reproduction of a message signal. The frequency responsive control circuit for each receiver is constructed in a manner such that it has a high degree of sensitivity to insure that it provides switching only in response to a particular one of the code signals.

While several specific embodiments of the principles of the present invention are illustrated in the accompanying drawing and described in detail in the following specification, it is to be understood that such embodiments are' by way of example and that various mechanical modifications may be made without departing from the spirit of the invention, the scope of which is limited only as defined in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a radio broadcasting system of the present invention;

FIG. 2 is a block diagram of one of the receivers included in the system of FIG. 1 according to the present invention;

FIG. 3 is a schematic diagram of a control circuit according to the present invention included in the receiver of FIG. 2;

FIG. 4 is a block diagram of an alternative embodiment of a receiver included in the broadcasting system of the present invention;

FIG. 5 is a schematic diagram of a speaker selection control circuit included in the receiver of FIG. 4; and

FIG. 6 is a schematic diagram of an alternative embodiment of the control circuit shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A radio broadcasting system according to the present invention includes a single transmitting station and a plurality of radio receivers for broadcasting a program, such as background music, on one channel to all receivers and periodic messages, such as commercials, on another channel to selected ones of the receivers. Selection is accomplished by generating coded tone signals along with the messages according to which receiver is intended to receive a particular message, the receivers, in turn, being provided with control means responsive to particular ones of the code signals. Referring now to the block diagram of FIG. I, the broadcasting system of the present invention includes a transmitter, designated generally at 10, for transmitting from an antenna 11 a program signal on one channel and periodic message signals on another channel. The standard transmitter mixer and output amplification stages are represented by block 12, the output of which is connected to antenna 11. Connected to the input of transmitter stages 12 are a program signal source 13, a message signal source 14, and a coding means 15 as shown in FIG. 1. In a particular application of the present invention the program signal consists of music, and block 13 thus would represent a conventional record player or tape recorder having the acoustical output signal thereof converted and translated through a noise suppressor, preamplifier and automatic volume control and applied to the input of the mixer included in block 12. In this same application the message signal consists of speech such as commercial messages and, therefore, block 14 would include a tape recorder or microphone having the acoustical output signal thereof translated through an amplifier to the input of the mixer included in block 12. A coding means 15 also is included in transmitter for generating a plurality of coded tone signals, each of a different frequency, which occur in time at a point corresponding to the beginning of selected ones of the message signals from block 14. The code signals from coding means are applied to the input of the mixer in transmitter stage 12, and each of the signals has a duration relatively much shorter than the normal duration of the periodic message signals provided by block 14.

In operation, the main carrier of transmitter 10 can be frequency modulated into two subcarriers corresponding to the program and message signals. Preferably, transmission of the program signal is at 67 kilocycles although a variation within the range of 59 to 75 kilocycles is not objectionable. Similarly, transmission of the message signals can be within the range of 37 to 47 kilocycles with transmission at 42 kilocycles being preferred. A plurality of codesignals can be generated in the range of to 35 kilocycles, with particular ones of the code signals occurring in time at a point corresponding. to the beginning of particular ones of the message signals. In one illustrative example, 19 code signals are generated within the foregoing prescribed range, and the frequency spacing was logarithmic wherein the spaces have a magnitude of 0.5

kilocycle in the lower portion of the range and a spacing of 1.0 kilocycle in the upper portion of the range. In addition, each of the code signals preferably has a duration of about three seconds.

Inasmuch as radio transmitter stations similar to the one described herein are readily familiar to those skilled in the art, the foregoing description is believed to be in sufficient detail. Equipment for taping messages and for recording the desired coded tones or signals on the tape alongside the message under control of a manually operated selector is readily commercially available for use in such radio transmitter stations.

The radio broadcasting system of the present invention further includes a plurality of radio receivers 20, and each receiver 20 has an antenna 21. In addition, the receivers 20 are of substantially identical construction as indicated by the use of the prime superscripts in FIG. 1. Each receiver 20 normally audibly reproduces the program signal from transmitter 10 and is adapted to also audibly reproduce the message signal from transmitter 10 upon switching of the receiver mode of operation. Each receiver is provided with control means responsive to a particular code signal from transmitter 10 for switching receiver operation and thereby resulting in audible reproduction of the message signal as will be described in detail hereafter. A preferred use of the system of FIG. I is in broadcasting background music to business establishments for convenience of the customers therein and for broadcasting periodic messages, such as commercials, to selected ones of the receivers. To this end a single radio receiver 20 can be located in one establishment and provided with several speakers positioned around the area in which the customers normally are present. A

number of different establishments can be serviced by the present system to be provided with the same background music but with different commercial messages, the number being equal to the number of code signals generated by transmitter 10. It also will be appreciated that the same commercial message is likely to be broadcast to all of the branches of a particular retain chain, and therefore the total number of receivers included in the system would be greater than the number of code signals. In other words, the capacity of transmitter 10 in terms of the number of code signals which can be provided is the constraint on the number of different commercial messages which can be transmitted in the system. The number of receivers for which the same commercial message is intended is limited only by geographic considerations and the radiation capability of transmitter 10.

A preferred form of receiver 20 for use in the broadcast system of the present invention is shown in the block diagram of FIG. 2. The other receivers for use in the system are identical in construction to receiver 20 shown in FIG. 2 with the exception that the frequency responsive portion of each is adjusted or modified so as to be activated by a particular one ofthe code signals from transmitter 10 as will be explained in detail hereafter. Referring now to FIG. 2, receiver 20 includes means for detecting the program, message and code signals from transmitter 10 comprising an antenna 21, connected to the input of a radio frequency amplifier and oscillator-mixer stage 22, the output of which is translated through an intermediate frequency amplifier stage 23 to the input of a limiter and discriminator stage 24. The program and message signals are audibly reproduced by means including audio amplifier stage 25, the output of which is connected to the input of each of a plurality of speakers designated 26 in FIG. 2. The stages of receiver 20 described so far are those of a standard or conventional radio receiver, and one receiver found readily adaptable for use in the present invention is available commercially under the designation Browning CRA.

In accordance with this invention, receiver 20 is adapted to normally audibly reproduce the program signal, but to be switched in operation to audibly reproduce the message signal when a particular one of the code signals from transmitter 10 activates a control means in receiver 20. Referring again to FIG. 2, receiver 20 also includes a first filtering means 27 for passing the program signal and rejecting the message signal. For convenience in illustration, filter means 27 is labeled with the frequency F, which is that of the program signal. The output of filter 27 is connected to the input of audio amplifier 25 through line 28 and a circuit designated 29 in FIG. 2. Circuit 29 includes conventional subcarrier amplifier, demodulator and squelch circuits. Receiver 20 further includes a second filtering means 31 for passing the message signal and rejecting the program signal. For convenience in illustration, filter 31 is labeled with the frequency F which is that of the message signal. The output of filter 31 is connected to the input of audio amplifier 25 through a line 32 and also through circuit 29.

Receiver 20 further includes switching means, designated generally at 34, for connecting either of the first or second filtering means 27 and 31, respectively,

to the receiver detecting means, in particular to line 35 connected to the output of limiter and discriminator stage 24. Receiver 20 finally comprises a control circuit 36 responsive to a particular one of the code signals from transmitter and which signal appears at thev output of the receiver detecting means for moving switching means 34 to cause receiver to a'udibly reproduce the message signal upon the occurrence of that code signal. The input of control circuit 36 is connected by means of a line 37, which in turn is connected to line 35, to the output of receiver limiter and discriminator stage 24. The output of control circuit 36 is operatively connected to switching means 34 as indicated by the dashed line 38 in FIG. 2 and which will be described in more detail hereafter. The subcarrier amplifier in circuit 29 is connected through line 39 to control circuit 36 to establish a holding circuit therein as will be described in detail hereafter.

FIG. 3 shows in detail one form of control circuit 36 constructed in accordance with this invention. Control circuit 36 comprises an amplifier including a triode 45 the output of which is rectified and coupled through a high resistance network to an electromechanical transducer for converting signals provided by circuit 36 into mechanical movement to operate switching means 34.

Triode 45 has cathode, grid and plate terminals 46-48,

respectively. The input of triode amplifier 45 is coupled to line 37, which is connected to the input of circuit 36, in the following manner. A tapped capacitance comprising the series combination of fixed capacitor 49 and variable capacitor 50 is connected between the input of amplifier 45 and ground. In particular, one terminal of fixed capacitor 49 is connected to grid terminal 47 of triode 45, and one terminal of variable capacitor 50 is connected to ground. The junction of capacitors 49 and 50 is connected by means of lead 51 to cathode terminal 46 of triode 45 and through a cathode resistor 52 to ground. An LC frequency responsive circuit 53 is connected across the series combination of capacitors 49 and 50. In particular, circuit 53 comprises an inductor 54 connected in parallel with the series combination of capacitors 55 and 56. The values of the components of circuit 53 are selected so that it is responsive to the frequency of a particular one of the coded tone signals generated by transmitter 10. The junction of capacitors 55, 56 is connected to the input terminal of circuit 36, here designated 36a, through a resistor 57 and the series combination of diodes 58, 59. Diodes 58, 59 are connected in opposed or back-to-back relation wherein the cathode of diode 58 is connected to resistor 57, the anodes of the diodes are connected together, and the cathode of diode 59 is connected to the input terminal 360 which, in turn, is connected to lead 37.

The output of triode amplifier 45 is rectified and coupled through a high resistance circuit to an electromechanical transducer at the output of control circuit 36. In particular, plate terminal 48 of triode 45, which also is connected through a resistor 60 and a lead 61 to a standard B+ voltage source, is connected through the series combination of a resistor 62 and capacitor 63 to the input of a full-wave rectifier comprising diodes 64 and '65. The anode of diode 64 is connected to one terminal of capacitor 63 and to the cathode of diode 65. Diode 65 is suitably biased by source of negative bias voltage 68. A full-wave rectified 7 signal in the form of a square wave is developed across a voltage divider comprising resistors 69, and 71 and is applied to .the input of a triode amplifier 72. Triode 72 has cathode, grid and plate terminals 73-75, respectively. Grid terminal 74 is connected to resistor 71 and through a capacitor 76 to ground. Cathode terminal 73 of triode 72 is connected directly to ground, and the output signal from amplifier 72 appears on plate 75, which is connected to one terminal of a relay winding 77, the other terminal of which is connected through a lead 78 to a standard B-lvoltage source. It is apparent, therefore, that an output signal from triode amplifier 72 causes energization of relay winding 77 which, in turn, provides a mechanical movement which is transmitted through the coupling indicated by the dashed line 38 to the arm of the switching means designated 34 in FIG. 2. It should also be apparent that other types of signal responsive switching means can be connected to the output of control circuit 36, such as to the output of amplifier 72, for switching the operation of receiver 20 from one of the filters 27, 31 to the other without departing from the spirit and scope of the present invention.

According to a preferred mode of the present invention, the coded tone signals will have a relatively short time duration, preferably about three seconds, which is significantly less than the duration of typical message signals broadcasted in the present system. Control circuit 36, therefore, is constructed to include a holding circuit for maintaining receiver 20 in a switched condition from one filter to the other upon the occurrence of a selected code signal for as long as the intended message signal is present.-Referring again to FIG. 3, it is seen that a switch 80 is included with one fixed contact 81 connected to the junction of resistors 69 and 70, and with a movable contact arm 82 normally engaging fixed contact 81 and mechanically coupled to relay winding 77 as indicated by the dashed line 83. A second fixed contact 84 of switch 80 is connected through a resistor 85 to one terminal of a coupling capacitor 86, the other terminal of which is connected to lead 39. It will be recalled from the discussion of the system of FIG. 2 that lead 39 is connected to the subcarrier amplifier in circuit 29 where the message signal will be available. A half-wave rectifier comprising diode 87 is connected to the junction of resistor 85 and capacitor 86, the cathode of diode 87 being connected to the junction, and diode 87 is suitably biased by virtue of the connection of the anode thereof to potentiometer 67 and negative bias source 68. A resistor 88 is connected in parallel with diode 87.

complished by the transmission of code signals by transmitter 10 at a time corresponding to the beginning of particular message signals. The message signals are broadcast on a second channel, for example at a frequency of 42 kilocycles, and advantageously can consist of advertising orother messages for the purpose of informing customers at particular establishments.

Each of the receivers shown in the system of FIG. 1 operates in thefollowing manner. Referring now to FIG. 2, switching means 34 normally is in the position shown in FIG. 2 whereby filter 27, which passes only the program signal, is connected in the transmission path of receiver 20, and filter 31 is therefore switched out of the circuit. The program signal from transmitter 10 is received by antenna 21 and translated through receiver stages 22-24 in a conventional manner, whereupon it is applied to the input of filter 27. Filter 27 is responsive to the frequency of the program signal, for example 67 kilocycles, and functions to pass that signal but to reject all other signals such as message signals or code signals. The output of filter 27 is applied through line 28 to the input of circuit 29 which in turn is connected to the input of an audio amplifier 25. The squelch circuit in circuit 29 is of known construction and functions to mute the signal transmitted to audio amplifier should thesignal from filter 27 decrease in amplitude below a desired level. The program signal is audibly reproduced and available at the output of the various speakers 26, which would be placed at spaced ment would hear music in the background for their pleasure and enjoyment.

Each receiver 20 has the capability of switching from the program signal to a mode of operation wherein it audibly reproduces a message signal. It is important to note, however, that reception of a particular message signal is only by a selected one or ones of receivers'20, and while that happens the remaining receivers in the system will continue to receive the program signal such as background music. Assume that the receiver shown in FIG. 2 has been receiving and audibly reproducing the aforementioned program signal and that. it is now about to receive 'and audibly reproduce a message signal intended for it and for a relatively short period of time whereupon the receiver will return to its original mode of operation wherein it reproduces the program signal. For a particular receiver 20 to reproduce a message signal two conditions must be met. A first condition is, of course, that the message signal be received by antenna 21, and a second condition is that the one discrete code signal intended for that receiver which will activate control circuit 36 is present also at antenna 21 and simultaneous with the beginning of the message signal.

The program, message and code signals are translated through receiver stages 22-24, and are applied simultaneously to the inputs of filter 27 through switching means 34 and of control circuit 36 through line 37. Assumingthat the present code signal is the particular one which will activate circuit 36, switching means 34 is moved from the position shown in FIG. 2 to the dotted line position whereupon line 35 is connected to the input of filter 31. Filter 31, in turn, functions to pass the message signal which, for example, has a frequency of 42 kilocycles, but to reject the program signal. The output of filter 31 is applied through line 32 to the input of circuit 29 and then through audio amplifier 25 and speakers 26, whereby the message such as a commercial is audibly reproduced. The continued transmission of the message signal through filter 31 for a predetermined time also completes a holding circuit conditioned by the operation of circuit 36 and indicated generally by means of line 39 in FIG. 2, to. maintain operation of control circuit 36 and hence' switching means 34 in the position shown whereby the detected signals are applied to the input, of filter 31 for the duration of the message signal. The code signal has a duration relatively much shorter than that of the message signal, and according to a preferred mode of the present invention the code signal lasts for about 3 seconds. Also, the code signal must be present for this to transient or spurious signals as will be described in more detail hereafter. Thus, during this mode of operation the background music or the program signal is temporarily interrupted at a particular receiver(s) and there is substituted an informative message such as a commercial. At the conclusion of the message; the receiver switches back to operation whereby the background music again is audibly reproduced because upon cessation of the message signal the aforementioned holding circuit ,is broken causing switching means 34 to return to its normal position as indicated in FIG. 2.

The broadcasting system of the present invention has several advantageous characteristics. The normal mode of operation of each receiver 20 in the broadcasting system is with switching means 34 in the position shown in FIG. 2 whereby the receiver 20 audibly reproduces the program signal. Switching of the receiver from audible reproduction of the program signal to audible reproduction of a message signal occurs only upon satisfaction of two conditions: the availability of a message signal and the simultaneous occurrence of a particular one of the code signals intended for that receiver. In addition, the time requirement on the code signal for operating circuit 36 prevents switching in response to transient or other spurious signals. Therefore, any malfunction in operation of either transmitter 10 or receiver 20 most likely will result either in continued reception of the program signal or cessation of receiver operation altogether. In other words, unintended switching of any receiver to reproduction of a message signal not intended for it will not occur in the system of the present invention. This is of considerable importance when receivers 20 are located respectively in different commercial establishments, and the advertising messages intended for one or more establishments would be completely foreign to and undesirable in any of the remaining establishments. In addition, the broadcasting system of the present invention has the capability of transmitting from a single receiver both a speakers'operatively connected thereto as shown in FIG. 2, which receiver is modified according to the present invention to include filters 27 and 31, together with control circuit 36.

The operation of control circuit 36, which was described briefly in the foregoing discussion of the overall system operation, now will be described in more detail. Referring to FIG. 3, the output of receiver limiter and discriminator stage 24 comprises the detected program, message and code signals and is applied through lead 37 to the input of circuit 36. These signals, in turn, are transmitted through diodes 58, 59 and resistor 57 to the input of tuned circuit 53. Circuit 53 is responsive to the frequency of only one of the plurality of code signals transmitted from transmitter 10, and upon the presence of that particular code signal, circuit 53 resonsates to provide a signal to the input of amplifier 45. The amplified signal then is full-wave rectified by diodes 64 and 65 resulting in a square wave signal across the divider network comprising resistors 69-71 which signal is applied to the input of amplifier 72. The output of amplifier 72, in turn, energizes relay coil 77 to move switching means 34 shown in FIG. 2 to the position whereby the line 35 is connected to the input of filter 31. The energization of relay winding 77 also causes movement of switch arm 82 away from contact 81 and into engagement with contact 84. A circuit thus is completed from line 39, which is connected to filter 31, through capacitor 86, resistor 85, and resistor 71 to the input of amplifier 72.

The message signal present at the subcarrier amplifier in circuit 29 is transmitted through line 39, half-wave rectified by diode 87, and is applied to the input of amplifier 72. The message signal is of sufficient amplitude so as to require only half-wave rectification as provided by the single diode 87. The rectified code signals of 3 seconds duration will have charged timing capacitor 76, and the voltage thereon is applied through tube 72 to energize relay winding 77. The time constant for this mode of operation is determined by the magnitudes of resistors 69 and 71 and capacitor 76 and is made the duration of the code signal, here three seconds, which is significantly greater than the duration of any transient or spurious signals which might be present in the system. This, in turn, prevents inadvertent switching of receiver operation from program to message signal reproduction in response to transient or spurious signals. Were no signal present on line 39, relay 77 would drop out within three seconds because of the discharge of capacitor 76 through resistors 71 and 85, the time constant determined the values of these three elements. Assuming a signal is present on line 39, however, amplifier 72 will continue to provide an output signal to energize winding 77 for the duration of the message signal transmitted through filter 31. Upon cessation of the message signal on line 39,

I capacitor 76 discharges and amplifier 72 no longer provides an output thereby de-energizing winding 77 with the result that switching means 34 in FIG. 2 returns to its normal position engaging contact arm 81.

When a relatively large number of receivers are included in the system of the present invention and when the code signals are relatively closely spaced in frequency, it is apparent that circuit 36 should have a flat or horizontal rather than an oblique or inclined frequency-voltage characteristic, so as to have a high degree of sensitivity. In other words, were circuit 36 to have an inclined frequency-voltage characteristic, code signals varying slightly in frequency from the discrete code signal frequency to which it is tuned could cause circuit 36 to provide an'undesired output signal. On the other hand, with a horizontal characteristic, the input signal must correspond in frequency precisely with the frequency to which circuit 53 is tuned so that a square wave of sufficient amplitude is developed for application to the input of amplifier 72. Otherwise the occurrence of an adjacent tone signal might cause an output signal to be generated by circuit 36, due to the inclined nature of the characteristic, and cause energization of relay 77 and an undesired switching from program to message signal reproduction.

Various features of circuit 36 contribute to the high degree of sensitivity between frequency andoutput voltage. The relatively loose degree of coupling provides a sharp frequency response. The provision of diodes 58 and 59 connected back-to-back in series with the input of tuned circuit 53, together with resistors 62, 69 and having relatively large magnitudes place relatively little load on the operation of tube 45. The tapped capacitance comprising fixed capacitor 49 and variable capacitor 50 connected to the input of tube tube 45 provides automatic feedback control wherein capacitor 49 equalizes the feedback to maintain a constant voltage in the tube plate circuit. Capacitor 50 is varied in magnitude to adjust the feedback in the circuit of tube 45 to compensate for manufacturing tolerances in the magnitudes of inductor 54 and capacitors 55, 56 of tuned circuit 53 and thus permit relatively precise tuning to a desired code signal.

According to a preferred mode of the present invention, various components of control circuit 36 have the following ratings or magnitudes:

Resistor 57 0.47 M

Capacitor 49 470 uuf Capacitor 50 0-680 uuf Resistor 62 0.47 M Capacitor 63 300 uuf Resistor 69 22 M Resistor 70 10 M Resistor 71 22 M Capacitor 76 0.l uf

Resistor 22 M Capacitor 86 300 uuf Diodes 58, 59 1N64 FIG. 4 illustrates a modification of each of the radio receivers included in the broadcast system of FIG. 1 according to a second embodiment of the present invention. In this embodiment the system of the present invention operates to broadcast a program signal to the public areas of a business establishment and periodic message signals only to a private area. Selection among a plurality of receivers is provided by the generation of coded tone signals from the transmitter. One advantageous use of the system according to this embodiment would be for a bank having a large number of branches, wherein it is desired to broadcast background music from a single transmitting station to the public areas of all the branches and periodic messages to only the managers office, employee area or other private area but not to the public area. As in the system according to the first embodiment, this system has the capability of transmitting different message signals to different receivers by virtue of the generation of a plurality of code signals, each of a different frequency, and of the inclusion of a frequency responsive control circuit within each receiver as will now be described in detail.

A receiver 100 constructed in accordance with this embodiment of the present invention includes an antenna 101 connected to the input of a radio frequency amplifier and oscillator-mixer stage 102, the output of which is translated through an intermediate frequency amplifier stage 103 to the input of a limiter and discriminator stage 104. The program and message signals available at the output of limiter and discriminator stage 104 are audibly reproduced by means of a subcarrier amplifier, demodulation and squelch stage 105, the output of which is translated through an audio amplifier 106, and applied through a switching means 107 to each of a plurality of speakers designated 108 in FIG. 4. The output of audio amplifier 106 is connected also through a line 109 to a speaker 110 for audibly reproducing message signals in an area which is separate or private from that area in which speakers 108 are located. While only one speaker 110 is shown, additional speakers could, of course, be connected to line 109. The foregoing components of receiver 110 between antenna 101 and audio amplifier 106 are arranged in a standard or conventional manner, and one receiver which was found to be readily adaptable to modification according to the present invention is marketed under the commercial designation Browning CRA.

According to the present invention, receiver 100 further includes a first filtering means 111 for passing the program signal and rejecting the message signal. In preferred form filter 111 is constructed to pass a frequency of about 67 kilocycles, and for convenience is designated F in FIG. 4. The output of filter 111 is connected to the input of amplifier squelch and demodulation state 105 through a line 112. Receiver 100 further includes a second filter means 113 for passing the message signal and rejecting the program signal. ln preferred form filter 113 is constructed to pass frequencies at about 42 kilocycles, and for convenience is designated F in FIG. 4. The output of filter 1 13 is connected to the input of squelch and demodulation stage 105 by means of a line 114. Receiver 100 further includes a switching means 115 for connecting either of the filtering means 111, 113 to the receiver detecting means. In particular, switching means 115 is 'shown in its normal position connecting the input of filter 112 through line 116 to the output of limiter and discriminator stage 104.

Receiver 100 further includes a control circuit 117 having an input connected to the output of the receiver detecting means, in this particular illustration the connection being provided by line 118 connecting line 116 transmitter. A holding circuit for control circuit 117 is provided including line 120 connected thereto and to the subcarrier amplifier in stage 105.

Receiver finally includes a speaker selection control circuit 121 having a first input connected through a line 122 to the line joining receiver stages and 106 and a second input connected through a line 123 to line 120 connected to receiver stage 105. The output of speaker selection circuit 121 is operatively connected to switching means 107 as indicated by the dashed line 124 in FIG. 4. Speaker selection circuit' 121 functions, briefly, to move switching means 107 and disconnect speakers 108 from the output of amplifier 106 in response to a signal to amplifier .106 applied through line 122 to one input of circuit 121 and to a signal fromamplifier squelch and demodulation circuit 105 applied through line 123 to the other input of circuit 121. Speaker 110 remains connected through line 109 to the output of audio amplifier 106 regardless of whether switching means 107 is in its normal position as shown in FIG. 4 or is moved therefrom i response to operation of circuit 121.

Receiver 100 shown in FIG. 4 represents one of a plurality of similar receivers which can be substituted for the receivers 20 in the system of FIG. 1. As in that system, there is transmitted to receiver 100 from a'single transmitting station a program signal, such as background music, on one channel, periodic message signals on another channel, and a plurality of code signals of different frequencies and occurring in time at the beginning of selected ones of the message signals. Receiver 100 operates in the following manner. The program, message and code signals are detected by antenna 101 and translated through the receiver stages 102-104. Switching means is in its normal position connecting line 116 to the input of filter 111, and in this mode of operation the program signal, preferably at a frequency of 67 kilocycles, is passed by filter 111 and audibly reproduced by receiver stages 105, 106 and speakers 108, 110. Switching means 107 in this mode of operation is in a position connecting speakers 108 to the output of audio amplifier 106, whereby the program signal, such as background music, is audibly reproduced in both the public and private areas of the business establishment.

Assume now that receiver 100 is detecting and reproducing the program signal. and there is transmitted to the receiver an intended message signal along with the particular code signal to which control circuit 117 will respond. The program, message and code signals are available on line 116, and the code signal is applied through line 118 to the input of control circuit 117, whereby the latter by virtue of the connection indicated at 119 in FIG. 4 causes movement of switching means 115 from its normal position to a position connecting line 116 to the input of filter 113. This will occur, of course, only if the code signal is of sufficient duration, such as three seconds, as in the case of control circuit 36. As a result, the program signal is no longer translated further through the receiver and the message signal is passed by filter 113 to receiver stages 105 and 106. The presence of the message signal in stage 105, and provided the code signal has the given duration, establishes through line 120 a holding circuit for control circuit 1 17 for the duration of the message signal.

Speaker selection circuit 121 is operated in response to the presence of the message signal on line 122 as will be explained, and operation is maintained by virtue of the connection through line 123. Selection circuit 121, in turn, through the operative connection indicated at 124 in FIG. 4, causes movement of switching means 107 from its normal position with the result that speakers 108 are disconnected from the output of audio amplifier 106. Speaker 110, however, remains connected to the amplifier output through the path provided by line 109. Speakers 108 remain disconnected from amplifier 106 for the duration of the message signal, and when the message signal is ended selection circuit 121 is deactivated and switching means 107 returned to its normal position connecting the output of amplifier 106 to speakers 108.

Receiver 100, therefore, functions to audibly reproduce a program signal, for example background music, simultaneouslyon two sets of speakers, one of which can be placed in the public area of a business establishment and the other in a private area. Receiver 100 functions further to audibly reproduce a message signal only on the set of speakers in the private area, for example in the manager's office of employee area of a bank. To this end, receiver 100 functions to disconnect the public area speakers from the receiver circuit in response to the presence of both the message signal and a particular code signal intended for that receiver. It will be appreciated that while only one speaker 110 is shown in FIG. 4, a plurality of similar speakers can be provided and positioned at selected locations in the designated private area. It should be apparent also that in a system including a large number of receivers similar to receiver 100, each having a control circuit 117 responsive to a different one of the plurality of code signals transmitted from the single station, a corresponding number of different message signals can be transmitted and properly routed to corresponding ones of the receivers where they are audibly reproduced on the speaker such as speaker 110 in the selected private area.

In a preferred form of receiver 100, filters 111 and 113 are responsive to frequencies of 67 kilocycles and 42 kilocycles, respectively, and thus are identical to filters 27 and 31, respectively, of receiver 20. Likewise, control circuit 117 is identical in construction to control circuit 36 shown in FIGS. 2 and 3.1 In particular, the holding circuit including line 120 is conditioned by the operation of circuit 117, and the code signal on line 118 must be present for at least a given time, such as three seconds, to develop a voltage level sufficient to maintain the holding circuit and switching means 115 in this position. In addition, line 122 in the circuit of FIG. 4 connects an input of selection circuit 121 to the output of stage 105 and line 123 connects another input to the subcarrier amplifier in stage 105.

A preferred form of speaker selection circuit 121 is shown in detail in FIG. 5. Circuit 121 comprises an input amplifier including a triode having cathode, grid and plate terminals 131-133, respectively. Line 122 from the output of receiver stage 105 is coupled through a capacitor 139 to grid terminal 132 of triode 130, and a tuned circuit comprising the parallel combination of a capacitor 135 and an inductor 136 is connected between grid terminal 132 and ground. Cathode 131 of triode 130 also is connected to ground through a resistor 137. A suitable bias voltage available on line 138 is applied through a resistor 139 to the plate terminal 133 of triode 130. Circuit 121 further comprises an output amplifier including a triode having cathode, grid and plate terminals 146-448, respectively. The output of amplifier 145, available on plate terminal 148, is connected to one terminal of a relay energizing winding 149, the other terminal of which connected through a line 150 to a suitable source of bias voltage rated B+. Relay winding 149 is mechanically coupled to switching means 107 shown in FIG. 4, as indicated by the dashed line 124. Energization of winding 149 causes movement of switching means 107 away from its normal position to thereby disconnect speakers 108 from line 109 connected to the output of audio amplifier 106.

The output of amplifier 130 is half-wave rectified and applied to the input of amplifier 145 through a network comprising a coupling capacitor 151, a diode rectifier 163, a resistor 152, a switch 153 including movable contact arm 154 and fixed contacts 155, 156, and a resistor 1S7. Coupling capacitor 151 is connected to plate terminal 133, switch arm 154 normally engages contact 155 thereby connecting resistor 157 through resistor 152 to capacitor 151, and a lead 158 connects resistor 157 to grid terminal 147 of triode 145. Grid= terminal 147 also is connected to ground through a capacitor 159. Relay winding 149 is mechanically coupled also to switch 153 as indicated by the dashed line 160 in FIG. 5, and contact 156 is connected through a resistor 161 to line 123 which, as shown in FIG. 4, is connected to line 120 and hence to the subcarrier amplifier in stage 105. Circuit 121 finally includes a source of negative bias voltage 162, one terminal of which is grounded and the other terminal of which is connected to the anode of diode 163, the cathode of which is connected to the junction of capacitor 151 and resistor 152.

Speaker selection circuit 121 operates in the following manner. The tuned circuit comprising capacitor 135 and inductor 136 is .constructed to resonate at a frequency within the modulation of the message signal, which according to a preferred mode of operation is 4,800 cycles. Thus, upon activation of control circuit 117 shown in FIG. 4 in response to the presence of the particular code signal, which causes filter 113 to be switching into the circuit to pass the 42 kilocycle message signal to the receiver amplifier, squelch and demodulation stage 105, the message signal is present on line 122. The signal is amplified by triode 130, rectified by diode 163, and appears as a square wave across the divided network comprising resistors 152 and 157. This square wave, in turn, is applied to the input of amplifier 145. The message signal is of sufficient amplitude so as to require only half-wave rectifimovement of switching means 107 to disconnect speakers 108 from the output of audio amplifier 106. At the same time, energization of winding 149 causes movement of switch arm 154 to engage fixed contact 156 by virtue of the mechanical connection 160 thereby establishing a holding circuit from the output of the amplifier in 105 through line 123, resistor 161, fixed contact 156, resistor 157 and lead 158 to the input of triode 145. Winding 149 thus remains energized for the duration of the message signal. Upon cessation of themessage signal, contact arm 154 of switching means 153 is returned to its normal position engaging fixed contact 155 and the holding circuit is thereby opened.

FIG. 6 shows a frequency responsive control circuit 180 which is an alternative embodiment of circuit 36 shown in FIG. 3. Circuit 180 includes a frequency responsive input portion 181 and a buffer stage 182. Buffer stage 182 includes a major portion of circuit 36 shown in FIG. 3, and for convenience in description the identical components are labeled with the same numbers but provided with a prime superscript. Input stage 181 includes a semiconductor amplifier in the form of field effect transistor 183 having gate, source and drain terminals 184 through 186, respectively. Input stage 181 further comprises a frequency responsive circuit 187 comprising an inductor 188 connected in parallel with the series combination of capacitors 189 and 190. Circuit 187 is connected at the junction of inductor 188 and capacitor 190 through a lead 191 to gate terminal 184 of transistor 183, and the junction of inductor 188 and capacitor 189 is connected to ground.

Input stage 181 further comprises a variable capacitor 192, one terminal of which is connected to the junction of capacitors 189 and 190 in circuit 187. The other terminal of capacitor 192 is connected through a resistor 193 to a lead designated 37 which, as in the embodiment of FIG. 3, is connected to line 35 and, hence, the output of receiverlimiter and discriminator stage 24. The junction of variable capacitor 192 and resistor 193 is connected through a feedback resistor 194 to source terminal 185 of transistor 183. Drain terminal 186 of transistor 183 is connected through the series combination of resistors 197 and 198 and a lead 199 to a source of B positive bias voltage. The junction of resistors 197 and 198 is connected through a resistor 200 to ground. A voltage output from transistor 183 is developed across a load resistor 201, one terminal of which is connected to ground and the other terminal of which is connected through a lead 202 to source terminal 185. The voltage developed on resistor 201 is applied through a lead 203 to grid terminal 47' of amplifier tube 45'. A compensating capacitor 204 is connected across cathode resistor 52'.

Circuit 180 operates in a manner similar to that of circuit 36 shown in FIG. 3. The magnitudes ofinductor 188 and capacitors 189, 190 are selected so that circuit 187 is responsive to a particular one of the coded tone signals. The feedback in the circuit of transistor 183 is adjusted by varying capacitor 192. This in turn compensates for manufacturing tolerances in the components of tuned circuit 187. Transistor 183 is a field effect transistor of the 2N38l9 type characterized, in

part, by having a relatively high input impedance. This, in turn, enhances the overall frequency-voltage sensitivity of circuit 180. The feedback in the circuit of transistor 183 is capacitive as compared to the inductive feedback in the circuit of tube 45 shown in FIG. 3.

When the particular coded tone signal is present on line 37', circuit 187 resonates and acts as an extremely narrow band filter. Transistor 183 and associated circuitry cause an appreciable multiplication or increase in the Q of resonant circuit 187. The resulting voltage developed across resistor 201 is applied to lead 203 to turn on tube 45'. Buffer stage 183 then operates in a manner identical to that portion of the circuit of FIG. 3.

While several specific embodiments of the present invention have been described in detail, this is done by way of illustration without thought of limitation.

I claim:

1. A radio broadcasting system comprising:

-a. a transmitter for transmitting a program signal on one channel and periodic message signals on another channel;

. coding means in said transmitter for generating plurality of code signals, each of a different frequency, and corresponding in time to the beginning of selected ones of the message signals;

. a plurality of radio receivers, each of said receivers normally audibly reproducingsaid program signal and each including means for switching operation to audibly reproduce said message signals and then switching operation thereof to resume reproduction of said program signal;

. control means in each of said receivers operativ in response to only a particular predetermined one of said code signals and connected to said switching means for switching operation of said receiver to audibly reproduce said messagesignal upon the occurrence of said particular one of said code signals, said control means including frequency responsive circuit means tuned to the frequency of said particular code signal and having means for providing a constant voltage at the output of said frequency responsive circuit means in response to the presence of said particular code signal frequency, rectifier means connected to the output of said frequency responsive circuit means, and a high resistance path connecting the output of said rectifier means to transducer means, and

. each of said receivers including first and second speaker means connected to the output thereof and wherein said apparatus includes means responsive to the presence of said message signal and to the operation of said control means for disconnecting one of said first or second speaker means from the-output of said receiver for the duration of said message signal.

2. Apparatus according to claim 1 wherein each of said receivers includes a first filter means for passing said program signal and rejecting said message signal, second filter means for passing said message signal and rejecting said program signal, switching means being connected in controlled relation to said control means for placing one of said filter means into operation in said receiver in response to the operation of said control means.

3. Apparatus according to claim 2 wherein said first filter means normally is in operation in said receiver and wherein said switching means removes said first filter means from operation and places said second filter means into operation in said receiver in response to the operation of said control means.

4. Apparatus according to claim 1 wherein the duration of each of said code signals is relatively much shorter than the duration of said message signals and wherein said control means includes holding circuit means for maintaining operation of said control means in response to the presence of said message signal and for the duration of said message signal.

5. Apparatus according to claim 1 wherein said control means includes timing means whereby the duration of said code signal must be greater than the time constant of said timing means for operation of said control means to be maintained.

6. in a radio broadcasting system wherein a transmitter transmits a program signal on one channel, periodic message signals on another channel, and a plurality of code signals of different frequencies and occurring in time at the beginning of selected ones of the message signals, a radio receiver comprising:

a. means for detecting the program, message and code signals;

means for audibly reproducing the program and message signals;

. first filtering means connected to said reproducing means for passing the program signal and rejecting the message signal;

second filtering means connected to said reproducing means for passing the message signal and rejecting the program signal;

e. switching means for connecting either of said first or second filtering means to said detecting means; and

f. frequency responsive control circuit means including a resonant circuit having an input and an output and tuned to the frequency of a particular one of said code signals, means for connecting the input of said resonant circuit directly to the output of said receiver detecting means, transducer means having an output operatively connected to said switching means, said control circuit means having means coupling the output of said resonant circuit to said transducer means with a horizontal frequency response at the frequency of said particular code signal so that the signal applied to said control circuit means must correspond in frequency precisely with the frequency to which said resonant circuit is tuned in order to operate said transducer means thereby causing movement of said switching means from said first filtering means to said second filtering means in response to the presence of only said particular one of said code signals.

7. A radio receiver according to claim 6 wherein said switching means normally connects the output of said detecting means to the input of said first filter means and is moved to the input of said second filter means in response to the operation of said control circuit means.

8. A radio receiver according to claim 6 wherein the duration of each of said code signals is relatively much shorter than the duration of said message signals and wherein said control circuit means includes holding circuit means coupled to said reproducing means for maintaining operation of said control circuit means for the duration of said message signal.

9. A radio receiver according to claim 6 wherein said control circuit means includes timing means whereby the duration of said code signal must be greater than the time constant of said timing means for operation of said control means to be maintained.

10. A radio receiver according to claim 6 including first and second speaker means connected to the output thereof and further including means responsive to the presence of said message signal and to the operation of said control circuit means for disconnecting one of said first or second speaker means from the output of saidreceiver for the duration of said message signal.

11. A radio receiver according to claim 6 wherein said means coupling the output of said resonant circuit to said transducer means comprises:

a. first amplifier means having an input and an output;

b. means including a variable capacitor for connecting the output of said resonant circuit to the input of said amplifier;

. rectifier means coupled to the output of said amplifier means;

. second amplifier means having an input and having an output connected to said transducer means; and

. a high resistance path connecting said rectifier means to said input of said second amplifier means.

12 In a radio broadcasting system wherein a receiver includes first and second transmission means to be in operation during corresponding first and second modes of reception and switching means for connecting the receiver detecting means selectively to either of said transmission means, a control circuit for operating said switching means in response to the reception of a particular code signal, said control circuit comprising:

a. a resonant circuit having an input and an output and tuned to the frequency of said particular code signal;

b. means connecting the input of said resonant circuit to the output of said receiver detecting means;

. first amplifier means having an input and an output;

. means including a variable capacitor for connecting the output of said resonant circuit to the input of said amplifier;

. rectifier means coupled to the output of said amplifier means;

f. second amplifier means having an input and an output;

. a high resistance path connecting said rectifier means to said input of said second amplifier means;

. capacitive timing means connected to the input of said second amplifier means whereby the duration of said code signal must be greater than the time constant of said timing means for operation of said control circuit to be maintained;

i. means for coupling said output of said second amplifier means to said switching means whereby the latter is moved in response to operation of said second amplifier means; and

j. holding circuit means coupled to one of said first and second transmission means and to the input of said second amplifier means for maintaining operation of said control circuit for the duration of one of said modes of reception upon cessation of said code signal.

13. A control circuit according to claim 12 wherein said means connecting the input of said resonant circuit to the output of said detecting means comprises the series combination of first and second diodes connected in opposed relation.

14. A control circuit according to claim 12 wherein said first amplifier means comprises a triode, wherein said means including said variable capacitor is connected between said resonant circuit and the grid of said triode, and wherein said means further comprises a fixed capacitor connected to said variable capacitor and means connecting the junction of said fixed capacitor and said variable capacitor to the cathode of said triode.

15. In a radio broadcasting system wherein a receiver includes first and second transmission means to be in operation during corresponding first and second modes of reception and switching means for connecting the receiver detecting means selectively to either ofsaid transmission means, a control circuit for operating said switching means in response to the reception of a particular code signal, said control circuit comprising:

a. a resonant circuit having an input and an output and tuned to the frequency of said particular code signal;

b. means including a variable capacitor connecting the input of said resonant circuit to the output of said receiver detecting means;

c. a field effect transistor having a gate terminal and an output terminal, said gate terminal being connected to the output of said resonant circuit;

d. amplifier means having an input and an output,

said input being connected to said transistor output terminal,

e. rectifier means coupled to the output of said am- I plifier means; f. transducer means having an input and an output;

g. a high resistance path connecting said rectifier means to said input of said transducer means; and h. means for coupling said output of said transducer means to said switching means whereby the latter is moved in response to a signal on said input of said transducer means. 

1. A radio broadcasting system comprising: a. a transmitter for transmitting a program signal on one channel and periodic message signals on another channel; b. coding means in said transmitter for generating plurality of code signals, each of a different frequency, and corresponding in time to the beginning of selected ones of the message signals; c. a plurality of radio receivers, each of said receivers normally audibly reproducing said program signal and each including means for switching operation to audibly reproduce said message signals and then switching operation thereof to resume reproduction of said program signal; d. control means in each of said receivers operative in response to only a particular predetermined one of said code signals and connected to said switching means for switching operation of said receiver to audibly reproduce said message signal upon the occurrence of said particular one of said code signals, said control means including frequency responsive circuit means tuned to the frequency of said particular code signal and having means for providing a constant voltage at the output of said frequency responsive circuit means in response to the presence of said particular code signal frequency, rectifier means connected to the output of said frequency responsive circuit means, and a high resistance path connecting the output of said rectifier means to transducer means, and e. each of said receivers including first and second speaker means connected to the output thereof and wherein said apparatus includes means responsive to the presence of said message signal and to the operation of said control means for disconnecting one of said first or second speaker means from the output of said receiver for the duration of said message signal.
 2. Apparatus according to claim 1 wherein each of said receivers includes a first filter means for passing said program signal and rejecting said message signal, second filter means for passing said message signal and rejecting said program signal, switching means being connected in controlled relation to said control means for placing one of said filter means into operation in said receiver in response to the operation of said control means.
 1. A radio broadcasting system comprising: a. a transmitter for transmitting a program signal on one channel and periodic message signals on another channel; b. coding means in said transmitter for generating plurality of code signals, each of a different frequency, and corresponding in time to the beginning of selected ones of the message signals; c. a plurality of radio receivers, each of said receivers normally audibly reproducing said program signal and each including means for switching operation to audibly reproduce said message signals and then switching operation thereof to resume reproduction of said program signal; d. control means in each of said receivers operative in response to only a particular predetermined one of said code signals and connected to said switching means for switching operation of said receiver to audibly reproduce said message signal upon the occurrence of said particular one of said code signals, said control means including frequency responsive circuit means tuned to the frequency of said particular code signal and having means for providing a constant voltage at the output of said frequency responsive circuit means in response to the presence of said particular code signal frequency, rectifier means connected to the output of said frequency responsive circuit means, and a high resistance path connecting the output of said rectifier means to transducer means, and e. each of said receivers including first and second speaker means connected to the output thereof and wherein said apparatus includes means responsive to the presence of said message signal and to the operation of said control means for disconnecting one of said first or second speaker means from the output of said receiver for the duration of said message signal.
 2. Apparatus according to claim 1 wherein each of said receivers includes a first filter means for passing said program signal and rejecting said message signal, second filter means for passing said message signal and rejecting said program signal, switching means being connected in controlled relation to said control means for placing one of said filter means into operation in said receiver in response to the operation of said control means.
 3. Apparatus according to claim 2 wherein said first filter means normally is in operation in said receiver and wherein said switching means removes said first filter means from operation and places said second filter means into operation in said receiver in response to the operation of said control means.
 4. Apparatus according to claim 1 wherein the duration of each of said code signals is relatively much shorter than the duration of said message signals and wherein said control means includes holding circuit means for maintaining operation of said control means in response to the presence of said message signal and for the duration of said message signal.
 5. Apparatus according to claim 1 wherein said control means includes timing means whereby the duration of said code signal must be greater than the time constant of said timing means for operation of said control means to be maintained.
 6. In a radio broadcasting system wherein a transmitter transmits a program signal on one channel, periodic message signals on another channel, and a plurality of code signals of different frequencies and occurring in time at the beginning of selected ones of the message signals, a radio receiver comprising: a. means for detecting the program, message and code signals; b. means for audibly reproducing the program and message signals; c. first filtering means connected to said reproducing means for passing the program signal and rejecting the message signal; d. second filtering means connected to said reproducing means for passing the message signal and rejecting the program signal; e. switching means for connecting either of said first or second filtering means to said detecting means; and f. frequency responsive control circuit means including a resonant circuit having an input and an output and tuned to the frequency of a particular one of said code signals, means for connecting the input of said resonant circuit directly to the output of said receiver detecting means, transducer means having an output operatively connected to said switching means, said control circuit means having means coupling the output of said resonant circuit to said transducer means with a horizontal frequency response at the frequency of said particular code signal so that the signal applied to said control circuit means must correspond in frequency precisely with the frequency to which said resonant circuit is tuned in order to operate said transducer means thereby causing movement of said switching means from said first filtering means to said second filtering means in response to the presence of only said particular one of said code signals.
 7. A radio receiver according to claim 6 wherein said switching means normally connects the output of said detecting means to the input of said first filter means and is moved to the input of said second filter means in response to the operation of said control circuit means.
 8. A radio receiver according to claim 6 wherein the duration of each of said code signals is relatively much shorter than the duration of said message signals and wherein said control circuit means includes holding circuit means coupled to said reproducing means for maintaining operation of said control circuit means for the duration of said message signal.
 9. A radio receiver according to claim 6 wherein said control circuit means includes timing means whereby the duration of said code signal Must be greater than the time constant of said timing means for operation of said control means to be maintained.
 10. A radio receiver according to claim 6 including first and second speaker means connected to the output thereof and further including means responsive to the presence of said message signal and to the operation of said control circuit means for disconnecting one of said first or second speaker means from the output of said receiver for the duration of said message signal.
 11. A radio receiver according to claim 6 wherein said means coupling the output of said resonant circuit to said transducer means comprises: a. first amplifier means having an input and an output; b. means including a variable capacitor for connecting the output of said resonant circuit to the input of said amplifier; c. rectifier means coupled to the output of said amplifier means; d. second amplifier means having an input and having an output connected to said transducer means; and e. a high resistance path connecting said rectifier means to said input of said second amplifier means.
 12. In a radio broadcasting system wherein a receiver includes first and second transmission means to be in operation during corresponding first and second modes of reception and switching means for connecting the receiver detecting means selectively to either of said transmission means, a control circuit for operating said switching means in response to the reception of a particular code signal, said control circuit comprising: a. a resonant circuit having an input and an output and tuned to the frequency of said particular code signal; b. means connecting the input of said resonant circuit to the output of said receiver detecting means; c. first amplifier means having an input and an output; d. means including a variable capacitor for connecting the output of said resonant circuit to the input of said amplifier; e. rectifier means coupled to the output of said amplifier means; f. second amplifier means having an input and an output; g. a high resistance path connecting said rectifier means to said input of said second amplifier means; h. capacitive timing means connected to the input of said second amplifier means whereby the duration of said code signal must be greater than the time constant of said timing means for operation of said control circuit to be maintained; i. means for coupling said output of said second amplifier means to said switching means whereby the latter is moved in response to operation of said second amplifier means; and j. holding circuit means coupled to one of said first and second transmission means and to the input of said second amplifier means for maintaining operation of said control circuit for the duration of one of said modes of reception upon cessation of said code signal.
 13. A control circuit according to claim 12 wherein said means connecting the input of said resonant circuit to the output of said detecting means comprises the series combination of first and second diodes connected in opposed relation.
 14. A control circuit according to claim 12 wherein said first amplifier means comprises a triode, wherein said means including said variable capacitor is connected between said resonant circuit and the grid of said triode, and wherein said means further comprises a fixed capacitor connected to said variable capacitor and means connecting the junction of said fixed capacitor and said variable capacitor to the cathode of said triode. 